When you think of diamonds, you likely envision brilliance, sparkle, and fire. However, some diamonds can exhibit a phenomenon that detracts from their natural beauty: the "fish-eye effect." This results in a dull, grey ring at the center of the diamond, which can be quite off-putting compared to the usual fiery sparkle we expect.

At first glance, you might mistake this grey spot for an inclusion, but in fact, it’s not part of the diamond itself. Instead, it’s a reflection of the girdle—the thin, outer edge of the diamond that divides the top portion (crown) from the bottom (pavilion)—which appears inside the table (the flat, top surface of the diamond).

The fish-eye effect is most commonly seen in diamonds with certain structural characteristics, including:

• A large table: The top surface of the diamond.
• A shallow pavilion: The lower part of the diamond, which can affect the way light enters and exits.
• A thick or unpolished girdle: This can distort light and cause reflections that result in a dull grey center.

When these features come together, the fish-eye effect becomes more apparent, leading to a dull appearance in the diamond's center. In some cases, this can even diminish the overall beauty of the stone.

However, not all diamonds with a fish-eye effect should be dismissed entirely. Some stones only show the grey spot when tilted at certain angles, which means you could still get a great deal for a diamond with this characteristic—especially if the stone is priced lower due to the flaw. Additionally, some diamonds with the fish-eye effect are prized for other qualities, such as color, which may make them a good option for buyers looking to prioritize other aspects of the stone.

The fish-eye phenomenon is most frequently seen in round diamonds due to the cutting angles, but even fancy-shaped diamonds can experience it if not cut optimally.

It’s important to note that a grading report won’t necessarily mention the fish-eye effect, as it’s a visual characteristic rather than an inherent flaw. This is why it’s crucial to inspect any diamond in person, from multiple angles and under different lighting conditions, before making a purchase. Doing so will help ensure you’re happy with your choice and avoid buying a diamond that might not live up to your expectations in terms of sparkle and brilliance.